The present invention relates to a method and apparatus for selectively oxidizing the carbon monoxide present in a mixture of gases, including hydrogen, carbon dioxide and water vapor. In particular, the present invention relates to an assembly and method for counteracting the poisoning effects of the selective oxidation catalyst active sites by carbon monoxide; to maintain the concentration of carbon monoxide in the outlet gas stream well below 10 parts per million (xe2x80x9cppmxe2x80x9d) and to periodically regenerate the selective oxidizer bed. Further, the present invention relates to an assembly for reducing fuel cell performance decay and polishing the fuel stream to eliminate residual traces of deleterious carbon monoxide.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Recently, efforts have been devoted to identifying ways to operate electrochemical fuel cells using other than pure hydrogen as the fuel. Fuel cell systems operating on pure hydrogen are generally disadvantageous because of the expense of producing and storing pure hydrogen gas. In addition, the use of liquid fuels is preferable to pure, stored hydrogen in some mobile and vehicular applications of electrochemical fuel cells.
Recent efforts have focused on the use of hydrogen obtained from the chemical conversion of hydrocarbon and oxygenated fuels into hydrogen rich gas. However, to be useful for proton exchange membrane fuel cells and other similar hydrogen-based chemical applications, these fuels must be efficiently converted to hydrogen with a minimal amount of undesirable chemical by-products, such as carbon monoxide (CO). The presence of such CO by-product greatly decreases the performance of the fuel cell and has a particularly detrimental effect on the anode of the fuel cell.
Conversion of hydrocarbons and oxygenated fuels such as methanol to hydrogen is generally accomplished through steam reformation in a reactor commonly referred to as a catalytic reformer. The steam reformation of methanol is represented by the following chemical equation:
CH3OH+H2O+heatxe2x86x923H2+CO2xe2x80x83xe2x80x83(1)
Due to competing reactions and thermodynamic limitations, the initial gaseous mixture produced by steam reformation of methanol typically contains from about 0.5% to about 20% by volume of carbon monoxide and about 65% to about 75% hydrogen, along with about 10% to about 25% carbon dioxide on a dry basis (in addition, water vapor can be present in the gas stream). The initial gas mixture produced by the steam reformer can be further processed by a shift reactor (sometimes called a shift converter) to increase the hydrogen content and to reduce the carbon monoxide content to about 0.2% to about 2%. The catalyzed reaction occurring in the shift converter is represented by the following chemical equation:
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(2)
Even after a combination of steam reformer/shift converter processing, the product gas mixture will have minor amounts of carbon monoxide and various hydrocarbon species, each present at about 1% or less of the total product mixture. A variety of conventional treatment processes may be employed to remove many of the hydrocarbon impurities generated during the steam reformer/shift converter process. However, such conventional treatment methods are generally incapable of reducing the carbon monoxide content of the gases much below 0.2%. Although this fuel processing was described for methanol as the fuel it is well known that other gaseous or liquid fuels, such as methane or gasoline, may be reformed to a hydrogen rich gas. Likewise alternatives to steam reforming, such as autothermal reforming, are also well known.
In low temperature, solid polymer proton exchange membrane fuel cell applications, which typically have an operating temperature of less than 100xc2x0 C., the presence of carbon monoxide in the inlet hydrogen stream, even at the 0.1% to 1% level, is generally unacceptable. In solid polymer electrolyte fuel cells, the electrochemical reaction is typically catalyzed by an active catalytic material comprising a noble metal, or noble metal alloys, such as platinum or platinum-ruthenium. In addition, other metals may be employed as a catalyst material, such as palladium or rhodium. Further, the noble metal may be promoted with metal oxides, such as iron oxide, cerium oxide, manganese dioxide, vanadium pentoxide, tungsten oxide, and the like.
However, carbon monoxide adsorbs preferentially to the surface of platinum on the anode, effectively poisoning the catalyst and significantly reducing the rate and efficiency of the desired electrochemical reaction. Thus, the amount of carbon monoxide in the hydrogen-containing gas mixture produced by a steam reformer/shift converter process for use in electrochemical fuel cells should be minimized, preferably to an amount significantly lower than the approximately 1% achieved using conventional steam reformation and shift conversion methods. The present selective oxidizing method and apparatus reduce the amount of carbon monoxide in a hydrogen-containing gas stream to a level suitable for use in low temperature electrochemical fuel cells, generally significantly less than 100 ppm.
In known selective oxidizing methods, it is believed that at least three competing reactions occur, which are represented by the following chemical equations:
1. The desired oxidation of carbon monoxide to carbon dioxide:
CO+xc2xdO2xe2x86x92CO2xe2x80x83xe2x80x83(3)
2. The undesired oxidation of hydrogen to water:
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(4)
3. The undesired reverse water gas shift reaction:
CO2+H2xe2x86x92H2O+COxe2x80x83xe2x80x83(5)
One of the most common selective oxidizer designs uses an adiabatic catalyst bed to react the carbon monoxide with oxygen supplied by an oxygen-containing gas (e.g., air). Catalyst loading, bed space velocity, and air flow are selected to control the temperatures in the bed so that bed size is minimized while the selectivity of the reaction to consume carbon monoxide is maximized.
To address these problems, prior art systems have been developed where at least two selective oxidizers are arranged in series with one another and operating at two distinct temperatures.
However, the foregoing prior art systems, such as U.S. Pat. No. 5,482,680, are not well suited for automobile applications where size and weight are of paramount concern. The reduction of power plant volume is of paramount concern. As a result, prior art systems with separate multiple selective oxidizer beds in series suffer from the disadvantages of increased weight, volume, complexity and the associated cost. The use of separate selective oxidizers adds weight, increases power plant volume and adds to power plant connective piping.
In addition, these systems operate exothermally thus requiring a separate heat exchanger to cool the stream prior to entering the fuel cell.
In view of the foregoing, an improved catalyst and selective oxidizer bed regeneration process is desired that employ systems that are much lighter in weight than prior art systems and less complex in design and less expensive to manufacture, operate and maintain. Further, there is a desire for a process that employs only a single selective oxidizer bed in series with the fuel cell. There is also a desire for such a process to permit the selective oxidizer to operate at a relatively low temperature to obviate the need for a separate heat exchanger to cool the oxidized gas prior to the fuel entering the cell. It is also desirable that a suitable process in a automobile environment be provided that is less expensive, more efficient and less complex to manufacture and operate.
The present invention preserves the advantages of prior art assemblies and processes as well as reducing the overall carbon monoxide level in a gaseous stream. In addition, it provides new advantages not found in currently available assemblies and processes, and overcomes many disadvantages of such currently known assemblies and processes.
An integrated fuel cell stack and selective oxidizer bed assembly is provided which includes a fuel cell stack assembly. The fuel cell stack assembly also includes a number of fuel cells. A fuel inlet manifold and fuel exhaust manifold are arranged in fluid communication with the fuel stack assembly for supplying to and exhausting from, respectively, the fuel supply in the fuel cells in the fuel stack assembly. The bed resides in said fuel inlet manifold. The bed includes catalyzed open cell foam with a heat exchange fluid conduit routed therethrough. Oxygen-containing gas is supplied into the bed via the input port. The temperature of the primary selective oxidizer bed is controlled by the fluid conduit in the bed in order to best reduce the carbon monoxide in the fuel.
In accordance with the present invention, the selective oxidizer is incorporated directly into the fuel inlet manifold. A primary advantage of positioning the selective oxidizer in the fuel inlet manifold is the reduction of the overall volume and weight of the entire power plant. The inventive process also includes the embedding cooling coils within a selective oxidizer bed of an open cell foam and introducing coolant through relatively flattened cooling coils to effectively control the temperature of the selective oxidizer bed. The coolest coils are located closest to a fuel outlet plenum. The temperature of a selective oxidizer bed is controlled by adjusting the temperature and/or flow rate of the coolant through the cooling coils or tubes during normal operation and regeneration.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide an apparatus and method of selectively removing carbon monoxide from a gaseous medium by oxidation.
Another object of the present invention is to reduce the weight and volume of a fuel cell power plant.
A further object of the present invention is to polish the fuel stream to minimize the concentration of carbon monoxide.
Yet another object of the present invention is to reduce the amount of power plant connective piping.
A concomitant object of the present invention is to design the apparatus of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.